1. Field of the Invention
The present invention relates to a method of forming semiconductor devices, and more specifically to a method of accurately controlling critical dimension (CD) in an etch process.
2. Discussion of Related Art
In a process for forming a semiconductor device, an etch process is used to form a pattern on a substrate. The etch process includes forming a photo mask, which transcribes the pattern into a semiconductor substrate, forming a material pattern on the semiconductor substrate. As patterns of the semiconductor device become more intricate, the accuracy with which the designed pattern is transcribed onto the substrate needs to increase. However, according to the properties of a layer, such as material thickness and exposure ratio, the pattern may be formed on the substrate having a different shape than the designed pattern. Further, an etch rate and a difference between the critical dimensions (CDs) of the etched pattern and designed pattern (i.e., dimensional displacement) may vary between different devices. A deformity of the pattern may be corrected by simulation or experimental pattern fitting. However, it is difficult to predict the dimensional displacement because the dimensional displacement is closely related to the exposure ratio. The exposure ratio is defined as a proportion of an area of an etched region to an area of the entire surface. It is difficult to measure the exposure ratio accurately, due to the complexity of patterns. In addition, conventional etch processes typically control CD using a sampling test. Thus, when intricate patterns are fabricated, samples for improving the etch method may be insufficient, such that CD cannot be accurately controlled. Even when a large number of devices are fabricated, it is difficult to precisely control CD because an etching aspect of different devices varies depending on the conditions of a preceding process, device, and etchant.
FIGS. 1A and 1B are schematic top plane views showing photo masks used for a photolithography process.
FIGS. 2A and 2B are cross-sectional views showing a method for forming the photo masks of FIGS. 1A and 1B.
Referring to FIGS. 1A and 1B, a photo mask comprises a transparent substrate 2 and opaque material patterns 4a and 4b. The opaque material patterns 4a and 4b are formed of a material such as chrome. A photo mask may have various ratios of opaque regions 4a and 4b and exposure regions 6a and 6b as occasion demands. Referring to FIG. 1A, a majority of the transparent substrate 2 is covered with opaque material 4a, such that the exposure ratio of region 6a is relatively low. Referring to FIG. 1B, however, the exposure ratio of region 6b is relatively high as compared to the photo mask of FIG. 1A. The two photo masks may be formed exhibiting different etching aspects depending on an exposure ratio in a fabrication process. In order to fabricate the photo mask, an opaque material and a photoresist pattern are formed on a transparent substrate 2. The opaque material is etched according to the photoresist pattern to form the opaque material patterns 4a and 4b. 
When a photo mask pattern having low exposure ratio is formed as illustrated in FIG. 2A, a photoresist pattern 8a formed on the transparent pattern 2 is etched during etching of the opaque material, thereby forming a deformed photoresist pattern 10a. The opaque material is etched during end point detection (EPD) time and over-etched during a predetermined time to remove any remaining opaque material from etched regions. The over-etch time is applied to an etching process in proportion to the EPD time. When the exposure ratio is low, EPD time is short because there is less opaque material as compared to photo masks having high exposure ratios. Therefore, the photoresist pattern sustains less etching damage during an etch process for forming an opaque pattern 4a. Further, the dimensional displacement L12, which is a difference between designed CDs L1 and etched CDs L2, is small.
Referring to FIG. 2B, when a photo mask pattern having a relatively high exposure ratio is formed, a comparatively large amount of opaque material is etched. Therefore, the EPD time and over-etch time are relatively long, and a photoresist pattern 8b is eroded. The etch process for forming the opaque pattern 4b is relatively long, and the sidewalls of the photoresist pattern 8b are eroded. Thus, the photoresist pattern 10b has a large difference between the designed CD L3 and the etched CD L4.
As stated above, the dimensional displacement of the photo mask pattern varies according to the exposure ratio. Variations in CD are difficult to predicted prior to performing the etching process because photo masks typically have unique patterns. Further, etching aspects vary depending on process conditions such as device parameters, etchant, preliminary process, and material layers used in the etching process. Therefore, an accurate pattern can be difficult to achieve if an over-etch time is linearly proportional to EPD time or if an experimentally achieved time is uniformly applied.